This invention is related, in general, to spectrometers, and more particularly to a monolithic diffraction spectrometer formed on a semiconductor chip.
Spectrometers are used in a variety of laboratory analysis and research instruments. Spectrometers break a beam of polychromatic incident light into multiple bands of monochromatic light. Since most materials have a characteristic combination of colors which are emitted or reflected, a spectrometer can be used to identify material type or composition of a light emitting or light reflecting target.
Many applications exist for small compact spectrometers which could be used for a fast, relatively accurate analysis of an incident light beam. For example, automotive fuel monitoring, spark color monitoring, and in-line process control equipment all could use an inexpensive color analysis device. Unfortunately, most commercial spectrometers use multiple optical elements such as lenses, reflectors, and diffraction gratings which are both bulky and difficult to align. One such spectrometer is described in U.S. Pat. No. 4,798,464 issued to Roy E. Boostrom. These mechanical and optical elements create a somewhat delicate instrument which cannot be used in many commercial applications. Also, additional electronic components had to be used to process the spectrometer output. It is believed that the physical separation between the diffraction grating and the processing components created many oportunities of noise, both electronic and light caused, to interfer with the diffraction signal. Further, the size of such a spectrometer limits the applications in which it can be used. Size, complexity, and additional cost associated with complexity make most available spectrometer designs suitable only for laboratory use.
In addition to spectroscopy, imaging devices such as color television cameras require color separation and detection. Color cameras require only three or four wavelength bands to be separated, and so are simpler than spectrometers. Conventional transmission filters have been used to filter polychromatic incident light into the desired three or four bands. The filtered light is then directed to an imaging device such as a charge coupled device (CCD). Because of the added complexity, diffraction grating technology has not been used with color cameras.
Most spectrometers use a diffraction grating to split the incident light beam into separate wavelengths. A diffraction grating is merely an opaque material having a number of transparent slits formed at regular intervals. As light is transmitted through the plurality of slits, each wavelength is bent, or diffracted, at a unique diffraction angle. The diffraction angle is a function of both the wavelength of light and spacing between the slits of the diffraction grating.
In the past, diffraction spectrometers have been made using diffraction gratings having a slit width which is large in comparison to the wavelength of light passing through it. Also, the spacing between slits is usually large with respect to the wavelength of light to be diffracted. Visible light has wavelengths in the range of 0.5 to 1.0 microns. Typical diffraction gratings used in diffraction spectrometers have slit width and slit spacing in the order of 2 to 10 microns. A 2 micron resolution is considered high precision due to the large diffraction angle which results. Until now, however, diffraction gratings with diffraction spacings and slit widths less than the wavelength of incident light have not been used.
Diffraction gratings are usually constructed with a constant pitch. That is to say, slit width and spacing over the entire diffraction grating are constant rather than variable. A slit width less than the wavelength of incident light causes the light to be completely dispersed as it passes through the diffraction grating. Thus, incident light having wavelength (.lambda.) equal to 1.0 micron will be completely dispersed when traveling through a diffraction grating or slit having a width of 0.7 microns. A diffraction spectrum formed using such a diffraction grating will not show any color bands of wavelengths larger than 0.7 microns because all of these wavelengths are dispersed before forming an image in the spectrum. Because constant diffraction spacing and slit width have been used in spectrometers, diffraction gratings slit width and spacing less than 1.0 micron have had little utility since they would undesirably filter signals.
Usually, a spectrometer requires a diffraction grating having a large number of slits. Also, the diffraction grating is located a significant distance from the means for detecting the diffracted spectrum. This combination allows a diffraction spectrum to form where each color in the spectrum has a relatively large intensity due to the large number of slits which make up each band in the spectrum. The light diffracted from each of the slits combines additively to produce a very intense spectrum which can be easily analyzed. However, this large spacing between the diffraction grating and the means for detecting the diffracted spectrum has made it impossible to integrate the diffraction grating on the same substrate as the means for detecting the diffracted spectrum.
Accordingly, it is an object of the present invention to provide a spectrometer formed on a single semiconductor chip.
A further object of the present invention to provide a spectrometer with reduced need for external optical elements.
Another object of the present invention is to provide a monolithic color imaging device.
Another object of the present invention is to provide a spectrometer which is relatively inexpensive.
A further object of the present invention is to provide a spectrometer which can be integrated with a number of conventional semiconductor processing techniques.